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Show CHAP. XIII. MORPHOLOGY. 434 ili h 11 never probably, sen~ lines of affin·i ti·e s.. We ws eba of affin' ities between tl: e tangle the inextricable b t when we have a dis~ members of any on e clads s; ut look to some un1 r nown tinct object I. n vi.e w, and oh no to make sure b ut slo w plan of creation, we may ope progress. h Seen that the members of Morp h ol o gy ·- We avde tl of their habi·t s of 11' .1r e, 1 · depen en Y . the same c ass, In . th general plan of theu organ~ resemble each other 1~1 e ·s often expressed by the isation. ~his resem ·"ance b~ saying that the several term "unity of tyJ?e ' ord·.c£' nt species of the class d 1n the 111ere parts an organs The whole subject is inc~uded under are homologous. f Mor hology. This lS the most the general name 0 t r natural' history, and may interesting d~partmen ~ What can be more curious be said to be Its very s~u . an formed for grasping, than that the hand ? .a roth~ leg of the horse, the that of a mole for ~Iggin~ the wing of the bat, should paddle of the porpoise, a: e pattern, and should all be constructed on ~ ethsamsame relative positions? include t ll e same bones ' 1n. . et d strongly on the hl' g h S H "l · e has 1ns1s e Geoffroy t. 1 au · · homologous organs: f 1 t · connex1on 1n d importance o rea Ive almost any extent in form an the parts may change to . connected together size and yet th ey always remaifni d .r h 0r instance, t e in t' he same or d er. We never nf ' th1'e thigh and leg, bones of the arm and forearm, ormoes can be given to H the same na vV tr.ansposed. ence . idely different animals. e the homologous bones ln wh t ucti·on of the mouths t 1 · n t e cons r . see the same grea aw 1 different than the Wl-of insects: what can be mo:e f a sphinx-moth, the mensely long sp·u a1 p robosCis bo and the great J·a ws curious folded one of a bee or ~g~erving for such difof a beetle ?-yet all these organ ' CHAP. XIII. MORPHOLOGY. 435 ferent purposes, are formed by infinitely numerous modifications of an upper lip, mandibles, and two pairs of maxillre. Analogous laws govern the construction of the mouths and limbs of crustaceans. So it is with the flowers of plants. Nothing can be more hopeless than to attempt to explain this similarity of pattern in members of the same class, by utility or by the doctrine of final causes. The hopelessness of the attempt has been expressly admitted by Owen in his most interesting work on the 'Nature of Limbs.' On the ordinary view of the independent creation of each being, we can only say that so it is ;-that it has so pleased the Creator to construct each animal and plant. The explanation is manifest on the theory of the natural selection of successive slight modifications,-· . each modification being profitable in some way to the modified form, but often affecting by correlation of growth other parts of the organisation. In changes of this nature, there will be little or no tendency to modify the original pattern, or to transpose parts. The bones of a limb might be shortened and widened to any extent, and become gradually enveloped in thick membrane, so as to serve as a fin ; or a webbed foot might have all its bones, or certain bones, lengthened to any extent, and the membrane connecting them increased to any extent, so as to serve as a wing: yet in all this great amount of modification there will be no tendency to alter the framework of bones or the relative connexion of the several parts. If we suppose that the ancient progenitor, the archetype as it may be called, of all mammals, had its limbs constructed on the existing general pattern, for whatever purpose they served, we can at once perceive the plain signification of the homologous construction of the limbs throughout the whole class. So with the mouths of insects, we have only to u 2 |
